The principal way in which such an electromagnetic switching device functions is explained with reference to FIGS. 1 to 3 using an example of a contactor. As shown in FIG. 1, such a switching device contains an electromagnet 1 with a magnet yoke 2, on which, for example, two magnet coils 4 for magnetic excitation are arranged. A magnet armature 6 associated with the magnet yoke 2 is mounted in a sprung manner, as a result of a resetting arrangement comprising two resetting springs 8 connected in parallel, in a housing 10 (only illustrated symbolically) of the switching device.
The magnet yoke 2, the magnet coil 4 and the magnet armature 6 form an electromagnetic drive of the switching device. The magnet armature 6 is connected in a force-fitting manner to a movable contact link 14 via a prestressed contact spring 12. Two fixed contact carriers 16 are associated with the movable contact link 14. The magnet armature 6 forms the actuator of the magnetic drive for the relative movement between the contact link 14 and the contact carrier 16.
The contact link 14 and the fixed contact carrier 16 are each provided with contact pieces or contacts 18. The switching contact formed by the movable contact link 14 and the fixed contact carrier 16 is located in the opened position (OPEN position). In this disconnected state, the contacts 18 are located at a distance s0 and the pole faces 20 and 60 of the magnet yoke 2 and the magnet armature 6, respectively, are at a distance of d=H. The resetting springs 8 are prestressed so that the magnet armature 6 is pressed against a stop 22 with a pretensioning or holding force F0 in the rest position of the OPEN position.
When the magnet coils 4 are switched on, the magnet armature 6 is set in motion in the direction toward the magnet yoke 2 counter to the action of the holding force F=F0 exerted by the resetting springs 8, as is illustrated in the figure by the arrows.
FIG. 2 now shows a situation in which touching contact is made between the contacts 18 for the first time, i.e. the magnet armature 6 has covered a travel path s0. At this point in time, the pole faces 20, 60 are at a distance of d=ds=H−s0. The further closing movement of the magnet armature 6 now continues to take place counter to the increasing spring forces exerted by the resetting springs 8 and in addition counter to the action of the likewise increasing spring force exerted by the contact spring 12 connected in parallel therewith. Since the spring force exerted by the prestressed contact spring 12 is considerably greater than the spring force exerted by the resetting spring 8, the total resetting force acting on the magnet armature 6 increases suddenly.
As things develop, the magnetic force acting on the magnet armature 6 becomes greater than the resetting force exerted by the resetting spring 8 and the contact spring 12, and the magnet armature 6 can move further in the direction toward the magnet yoke 2 until finally, as is illustrated in FIG. 3, it rests with its pole faces 60 on the pole faces 20 of the magnet yoke 2 in an end or rest position (d=0).
The associated force profile is plotted in FIG. 4. In this figure, the resetting force F exerted on the magnet armature 6 by the resetting springs 8 and the contact spring 12 is plotted against the distance d between the pole faces 60, 20 of the magnet armature 6 and the magnet yoke 2. The curve shows that the resetting springs 8 (FIG. 1) exert the holding force F0 in the OPEN position. If current is flowing through the magnet coils 4, the magnet armature 6 is moved under the action of the attraction force exerted by the electromagnet 1 and counter to the action of the resetting springs 8 in the direction toward the pole faces 20 of the magnet yoke 2. With this movement, the resetting force F exerted in the opposite direction on the magnet armature 6 increases linearly with the increasing length contraction of the resetting springs 8, corresponding to the sum of the spring constants of the resetting springs 8. At the distance d=ds, the contacts 18 come into touching contact with one another, and the resetting force F acting on the magnet armature 6 increases suddenly as a result of the prestressed contact spring 12 being connected.
The holding force F0 exerted on the magnet armature 6 in the OPEN position in this position protects the switching device against undesired closing in the event of external mechanical oscillation or impact loading. Over the entire path covered between d0 and ds, the magnet armature 6 therefore always needs to overcome the resetting force F exerted by the resetting springs 8, which resetting force increases successively starting from a finite value (holding force F0) required for mechanically securing the magnet armature 6 in the OPEN position.
In order nevertheless to achieve short switching times (high closing forces), it is therefore necessary to design and dimension the magnet system 2, 4, 6 in such a way that the magnetic force acting on the magnet armature 6 is considerably greater than the resetting force exerted by the resetting springs 8. One disadvantage is the continuous increase in the resetting forces over the entire working range (magnet travel). This results in relatively high, unnecessary forces, which need to be overcome by a magnet drive which is designed to have a correspondingly higher power.